Phytoplankton succession and nitrogen utilization during the development of an upwelling bloom

S. E. Fawcett*, B. B. Ward

ABSTRACT: Diatoms typically constitute the majority of large phytoplankton in coastal systems, where they dominate upwelling conditions. The exact mechanism for diatom success has never been explicitly demonstrated, and we hypothesize that it is attributable to their early, rapid response to newly upwelled nitrate. This strategy allows diatoms to grow quickly and consume a disproportionate fraction of the available nutrients. To test this hypothesis and investigate differential growth of phytoplankton during the early stages of upwelling, an upwelling event—and the subsequent phytoplankton bloom—was simulated in a mesocosm experiment in Monterey Bay. Nitrate, ammonium and inorganic carbon uptake by 3 size fractions (0.7–5 µm, 5–20 µm, >20 µm) was measured daily through stable isotope tracer incubations. The particulate nitrogen and carbon biomass was initially dominated by the smallest phytoplankton, but the largest fraction increased most rapidly to dominate the biomass and nitrogen and carbon assimilation after a few days. All size fractions achieved similar maximum specific nitrate uptake rates (VNO3), but this occurred most rapidly and was maintained longest by the largest fraction. This initial acceleration of VNO3 appears to be the mechanism by which diatoms exploit upwelling conditions. All size fractions assimilated carbon and nitrogen in a nearly constant ratio of ~6.3 as the assemblage returned to balanced growth. Pigment measurements and light microscopy independently documented changing phytoplankton abundance, with diatoms demonstrating a characteristic pattern of succession. Changes in diatom diversity and assemblage composition occurred early, providing evidence of a faster response than observable changes in nitrate or biomass implied.